tying the knot

Tying the knot

Compilers usually have one or more data structures known as symbol tables, which are mappings from symbols to information about the symbol in question. GHC avoids symbol tables; instead, a symbol contains all information about itself. Thus, the data types for Haskell entities (Id, TyVar, TyCon, DataCon, and Class) form an immutable cyclic data structure, where everything points to everything else. This makes it very convenient for the consumer, because there are accessor functions with simple types, such as idType :: Id -> Type.

The downside of these cyclic data structures is that they are difficult to update. This has two implications:

we have to construct this graph in one go using a technique called tying the knot (since we can't update the graph after the fact--it's immutable!) and

if any of the data in the graph ever becomes out-of-date (as can occur when typechecking hs-boot loops), we have to throw out all of the in-memory data structures and rebuild the graph from scratch.

How this knot tying works is a dark corner of GHC, but hopefully this wiki page will shed some light on the matter.

Status

Knot-tying is all intimately tied up with the treatment of hs-boot files, so those tickets are listed here.

Practical advice

Add a pprTrace to a type environment, and have GHC spin into a loop or panic? This may be because you are forcing a thunk too early. Try printing out the unique keys of the environment instead, or moving the trace later.

If you really need to add pprTrace statements inside the knot, consider trying the advice here.

Consider using mkNaked**** instead of the usual functions if you are within the knot-tying code

Graph representation versus interface representation

Most type-checker entities which form the graph representation have a sister representation which is not cyclic, uses symbol tables, and suitable for serialization into an interface file. Here are some of the main correspondences:

In Type, the type constructor application contains the full TyCon which contains everything we could possibly want to know about the type constructor (e.g., if it is a synonym, what its unfolding is). In IfaceType, the application points to a stub data structure IfaceTyCon which only records the IfExtName of the TyCon in question (which is just a Name); to find out more information, we will have to go out and lookup this IfExtName in the symbol table associated with the interface type.

Converting from graph to interface representation. The primary functions which turn ModDetails into ModIface are mkIface (when we generated code) and mkIfaceTc (when we did not generate code.) There is also tyThingToIfaceDecl (which does the obvious thing) and toIface***** functions. These functions are all relatively straightforward: since all graph representations record globally unique Names identifying them, all we need to do is drop the extra information and preserve only the Name.

Converting from interface to graph representation. This process is referred to as typechecking the interface (in TcIface). Unlike the conversion to interface format, which is essentially pure, conversion from the interface format involves some global state: the existing graph of type checking entities which we are going to resolve Name references to. Generally speaking, there is a *unique* such graph, such that every Name maps to a unique TyThing in the graph, spanning over all of the typechecked entities GHC could possibly know about. In the common case, the only reason we typecheck an interface is to (lazily) load its entities into this global map.

Tying the knot when loading interfaces

Consider the following Haskell file:

dataT=MkTSdataS=MkST

When we serialize this into an interface file, we end up with two IfaceDecls: one for T and one for S, both of which contain names to refer to one another. But when it comes time to construct the TyCon representing these types, we are in trouble: the TyCon for T has a DataCon whose Type needs to refer to the TyCon for S, which in turn refers to the TyCon for T: we have a cycle. How can we tie the knot in order to let these two TyCons refer to each other? The idea is that we lazily defer typechecking the details about the TyCon (in particular, its DataCons) until we have populated the type environment for this interface with all of the TyThings (i.e., the TyCon) which we may refer to locally.

There are three parts to this:

First, typecheckIface in TcIface typechecks all of the IfaceDecls in the ModIface, and then writes them into a mutable variable which makes them available to other typechecking code to tie the knot:

-- Typecheck the decls. This is done lazily, so that the knot-tying -- within this single module work out right. In the If monad there is -- no global envt for the current interface; instead, the knot is tied -- through the if_rec_types field of IfGblEnv ; names_w_things <- loadDecls ignore_prags (mi_decls iface) ; let type_env = mkNameEnv names_w_things ; writeMutVar tc_env_var type_env

Second, the actual process of typechecking these declarations is done lazily using the forkM function (which unsafely converts a monadic typechecking operation into a thunk).

Finally, in tcIfaceGlobal in TcIface, whenever we would try to get the TyThing for a Name that is locally defined in the interface, we look it up instead in the mutable variable. Because this call to tcIfaceGlobal is suspended lazily, it won't get called until the mutable variable is populated with all the things we need.

The net effect is that at the point when we loadDecls the declarations, we have a list of Names and unevaluated TyThing thunks, which we write into the global environment. Later, when we actually force the TyThing thunk, the suspended typechecking computation goes ahead and looks up the thunk in the environment, which has since been updated with the thunks we need.

Variation: tying the knot when typechecking mutually recursive interfaces. Sometimes, recursive declarations can be spread out across several hi files (due to an hs-boot loop). In this case, laziness plays a similar role; however, instead of consulting per-interface mutable variable, the typechecking process consults the External Package State (EPS) (in the case of ghc -c) or the Home Package Table (HPT) (in the case of ghc --make).

The external package state is a mutable variable which stores a mapping from Names to TyThings for all the externally loaded symbols we know about (in the case of ghc -c, it also stores symbols from the home package as well). Whenever you call loadInterface, this variable is updated with the new symbols from that interface file. To tie the knot over multiple interface file, laziness plays a critical role: we first add thunks representing all of the declarations to the EPS without doing any interface typechecking for both interfaces; then when we force the thunk the names can be found by consulting the EPS.

The home package table is a mapping containing all of the ModDetails of modules from the home package which we are currently compiling. It's kept separate from the EPS because we often need to remove entities from this environment (e.g., we are in a GHCi session and we update a module.) Because these ModDetails are preserved from compilation to compilation (unlike ghc -c, where the EPS is reconstructed from scratch every invocation of GHC), we may have some ModDetails in the HPT which are linked against incomplete TyThings provided from typechecking an hs-boot file. Once we compile the real hs file for the hs-boot file, it's necessary to *re-typecheck* all of the interfaces in the module loop, so that they end up pointing to the most up-to-date TyThings; this is done by simply throwing out all of the old ModDetails from the HPT and then reloading them back in. (retypecheckLoop in GhcMake).

Tying the knot when typechecking a module

As we typecheck Haskell source code, we produce TyCons and other type-checking entities. If some declarations are mutually recursive, then we need to similarly tie the knot. There are two primary cases when this can occur:

A mutually recursive set of source declarations. GHC simply arranges for every declaration in a mutually recursive set of declarations to be typechecked "all at once." For example, tcTyClDecls in TcTyClsDecls uses fixM to refer to the resulting type declarations, so they can be placed in the environment when we typecheck these very type declarations.

An hs file which implements an hs-boot file. This is the trickiest case of knot-tying during type checking, so let's look at a particular example:

Like before, T and S form a mutually recursive loop; the difference is this time it is done through an hs-boot file. At the point in time when we typecheck A.hs, we would like the TyCons for T and S to be mutually recursive.

However, this leads to a very intriguing requirement: when we typecheck the interface for B.hi, we must tie the knot with the local type environment (while typechecking.) Thus, rather than a mutable variable for the interface, we need to refer to a mutable variable for the current type-checking session. This variable is tcg_type_env_var in TcGblEnv. It is updated at various points during the typechecking session, including when we setup the type environment in tcTyClDecls (tcExtendRecEnv does the dirty work.)

This leads to another complication with ghc --make: just how we must retypecheck the interface files after we finish typechecking a module loop, we must also retypecheck the interface files BEFORE we start typechecking, so that the knot-tying can take place. Failure to do this lead to #12035 (closed). (Actually, hypothetically you could remove the later retypecheck, but we need it so that we can get up-to-date unfoldings, which aren't computed until after we run the optimizer, which is after all the thunks have been forced.)

All of the bits and bobs

GHC's present knot-tying story is a bit hard to understand. It consists of at least the folowing components (if this seems like a random jumble, it's because it is!):

if_rec_types :: Maybe (Module, IfG TypeEnv) in IfGblEnv. This field affects how typechecking interfaces works. Operationally, when this variable is Just (mod, get_tyenv), any Name from mod is typechecked by looking it up in a mutable variable that is accessible via get_tyenv. The source of this mutable variable depends on how the interface monad is run (initIfaceCheck, initIfaceTcRn, initIfaceTc).

initIfaceCheck :: HscEnv -> IfG a -> IO a. This initializes the interface monad with "no useful info at all." According to its name, this is only supposed to be used when we are checking if an old interface is up-to-date. It fills if_rec_types depending on if hsc_type_env_var is set. In practice, there are a few uses:

checkOldIface: We are in the recompilation manager and are trying to decide if an interface is up-to-date. Lives in IfG because we invoke loadInterface to get the hashes which we are going to compare against.

genModDetails: We got an up-to-date interface (from checkOldIface!) and now we want to typecheck it into a ModDetails so it can be put in the HPT in make mode. (The HomeModInfo is ignored in one-shot mode.)

typecheckLoop: In make mode, we finished typechecking an hs file which completes a loop: now we need to retypecheck the loop of interfaces so that the HPT properly points to the right place

A few miscellaneous calls getLinkDeps and abiHash which just want to get the ModIface via loadInterface.

initIfaceTcRn :: IfG a -> TcRn a. This initializes the interface monad while typechecking. In this case, if_rec_types is filled using tcg_type_env_var, which is updated in the course of typechecking. This is the preferred way to load things into the EPS because it will knot-tie correctly with the ongoing typechecking computation.

tcg_type_env_var in TcGblEnv, which is used to initialize if_rec_types when you call initIfaceTcRn. It is updated as we typecheck declarations. It itself is initialized by hsc_type_env_var (in the case of one-shot mode; in make mode any recursive references must be in the HPT already), or just a fresh variable otherwise.

hsc_type_env_var in HscEnv, which is used to initialize tcg_type_env_var in initTcRn and if_rec_types in initIfaceCheck. It is set in exactly one place, hscIncrementalCompile. We need to setup the type variable here because checkOldIface can load up home modules (this is not the case for make mode; those interfaces are already in the HPT), and we need the "right" type variable to already be fed in when we construct the thunks.